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Above polar ice sheets, atmospheric water vapor exchange occurs across the planetary boundary layer (PBL) and is an important mechanism in a number of processes that affect the surface mass balance of the ice sheets. Yet, this exchange is not well understood and has substantial implications for modeling and remote sensing of the polar hydrologic cycle. Efforts to characterize the exchange face substantial logistical challenges including the remoteness of ice sheet field camps, extreme weather conditions, low humidity and temperature that limit the effectiveness of instruments, and dangers associated with flying manned aircraft at low altitudes. Here, we present an unmanned aerial vehicle (UAV) sampling platform for operation in extreme polar environments that is capable of sampling atmospheric water vapor for subsequent measurement of water isotopes. This system was deployed to the East Greenland Ice-core Project (EastGRIP) camp in northeast Greenland during summer 2019. Four sampling flight missions were completed. With a suite of atmospheric measurements aboard the UAV (temperature, humidity, pressure, GPS) we determine the height of the PBL using online algorithms, allowing for strategic decision-making by the pilot to sample water isotopes above and below the PBL. Water isotope data were measured by a Picarro L2130-i instrument using flasks of atmospheric air collected within the nose cone of the UAV. The internal repeatability for δD and δ18O was 2.8 ‰ and 0.45 ‰, respectively, which we also compared to independent EastGRIP tower-isotope data. Based on these results, we demonstrate the efficacy of this new UAV-isotope platform and present improvements to be utilized in future polar field campaigns. The system is also designed to be readily adaptable to other fields of study, such as measurement of carbon cycle gases or remote sensing of ground conditions.

Reconstructions of past West Antarctic Ice Sheet (WAIS) climate rely on the isotopologues of water recorded in ice cores which extend the local surface temperature record back tens of thousands of years. Here, we utilize continuous flow sampling and novel back-diffusion techniques with the WAIS Divide ice core (WDCobs) to construct a seasonal record of the δ18O value of the precipitation (δ18Op) at the time of deposition from 1980-2000. We then use a water isotope enabled global climate model, iCESM1, to establish seasonal drivers of WAIS climate and of δ18Op variability at the WAIS Divide location to compare with the WDCobs and MERRA2 reanalysis data. Our results show that the WAIS seasonal climate variability is driven by the position and strength of the Amundsen Sea Low (ASL) caused by variations in the Southern Annual Mode and the two Pacific-South American patterns (PSA1 and PSA2). The largest year-to-year seasonal δ18Op anomalies at the WAIS Divide location occur with respect to PSA2 during austral winter (JJA) as a result of an eastward displacement of the ASL that shifts the associated onshore winds towards the Weddell Sea, reducing temperatures and precipitation near the WAIS Divide location. Additionally, the iCESM1 experiment suggests that changes to the moisture path from the source to the WAIS Divide location is an important driver of seasonal WDCobs δ18Op variability. This work highlights the potential of using a single ice core to reconstruct past WAIS climate at seasonal timescales.

An ice core drilled on the Renland ice cap in east-central Greenland contains a continuous climate record dating through the last glacial period. The Renland record is valuable because the coastal environment is more likely to reflect regional sea surface conditions compared to inland Greenland ice cores that capture synoptic variability. Here we present the δ18O water isotope record for the Holocene, in which decadal-scale climate information is retained for the last 8 kyr, while the annual water isotope signal is preserved throughout the last 2.6 kyr. To investigate regional climate information preserved in the water isotope record, we apply spectral analysis techniques to a 300-year moving window to determine the mean strength of varying frequency bands through time. We find that the strength of 15–20-year δ18O variability exhibits a millennial-scale signal in line with the well-known Bond events. Comparison to other North Atlantic proxy records suggests that the 15–20-year variability may reflect fluctuating sea surface conditions throughout the Holocene, driven by changes in the strength of the Atlantic Meridional Overturning Circulation. Additional analysis of the seasonal signal over the last 2.6 kyr reveals that the winter δ18O signal has experienced a decreasing trend, while the summer signal has predominantly remained stable. The winter trend may correspond to an increase in Arctic sea ice cover, which is driven by a decrease in total annual insolation, and is also likely influenced by regional climate variables such as atmospheric and oceanic circulation. In the context of anthropogenic climate change, the winter trend may have important implications for feedback processes as sea ice retreats in the Arctic.

Observed variability in summer surface snow isotopic composition (δ18O, δD) cannot solely be explained by precipitation events. This variability however, influences the overall summer isotope signal that is archived in ice cores. It is therefore important to explain the origin of such post-depositional modifications of the snow isotope signal to ensure an optimal interpretation of ice core isotope records. The continuous exchange of humidity between the atmospheric vapor reservoir and the snow surface through sublimation and deposition could be a key process. Yet, in the past, the surface humidity flux has been disregarded as influential for the formation of the isotope signal in snow based on the assumption of the absence of isotopic fractionation during sublimation. Here we show evidence of isotopic fractionation during snow sublimation through a combination of laboratory experiments, in-situ observations from the Greenland Ice Sheet, and snow surface modeling. We document substantial isotopic enrichment in the uppermost centimeters of snow induced by sublimation and find that the in-situ observed summer season temporal evolution of the snow surface isotopic composition (in between precipitation events) can be attributed to surface humidity fluxes. We further discuss the nature and the underlying physical process of fractionation during sublimation. Our results lead to an improved process understanding and necessitate the implementation of fractionation during the sublimation process in isotope-enabled climate models.

Disentangling the drivers of mean annual temperature change in Antarctica requires an understanding of seasonal temperature change. A high-resolution climate record capable of resolving summer and winter seasons could address long-standing questions about the role of orbitally-driven insolation in driving Antarctic mean-annual temperature change. Here, we present a continuous record of water isotope ratios from the West Antarctic Ice Sheet (WAIS) Divide ice core that reveals both summer and winter temperature change though the last 11,000 years. This novel record shows that summer temperatures increased through the early-to-mid Holocene, reached a plateau at 4 to 2 ka, and then decreased to the present. The observed changes are explained primarily by changes in maximum summer insolation. In the early to mid-Holocene, additional summer warming results from the retreat and thinning of the WAIS. The magnitude of summer temperature change constrains the lowering of the WAIS surface to less than 100 m since the early Holocene, consistent with geologic records. Importantly, annual mean temperatures cannot be fully explained by orbital forcing and ice sheet elevation change alone; in the early Holocene, large wintertime temperature excursions overwhelm the summer signal. These winter excursions indicate that regional heat transport anomalies, rather than local thermodynamics, can dominate the annual mean.

Antarctic paleotemperatures It has been widely thought that East Antarctica was ∼9°C cooler during the Last Glacial Maximum, close to the ∼10°C difference between then and now determined independently for West Antarctica. Buizert et al. used borehole thermometry, firn density reconstructions, and climate modeling to show that the temperature in East Antarctica was actually only ∼4° to 7°C cooler during the Last Glacial Maximum. This result has important consequences for our understanding of Antarctic climate, polar amplification, and global climate change. Science , abd2897, this issue p. 1097

Data from the South Pole ice core (SPC14) are used to constrain climate conditions and ice-flow-induced layer thinning for the last 54,000 years. Empirical constraints are obtained from the SPC14 i...

The δD and δ18O values of water are key measurements in polar ice-core research, owing to their strong and well-understood relationship with local temperature. Deuterium excess, d, the deviation from the average linear relationship between δD and δ18O, is also commonly used to provide information about the oceanic moisture sources where polar precipitation originates. Measurements of δ17O and “17O excess” (Δ17O) are also of interest because of their potential to provide information complementary to d. Such measurements are challenging because of the greater precision required, particularly for Δ17O. Here, high-precision measurements are reported for δ17O, δ18O, and δD on a new ice core from the South Pole, using a continuous-flow measurement system coupled to two cavity ring-down laser spectroscopy instruments. Replicate measurements show that at 0.5 cm resolution, external precision is ∼0.2‰ for δ17O and δ18O, and ∼1‰ for δD. For Δ17O, achieving external precision of 17O variations from the South Pole shows significant millennial-scale variability, and is correlated with the logarithmic formulation of deuterium excess (dln), but not the traditional linear formulation (d).

With recent advances in analytical techniques, water stable isotope ratios can be measured in astounding detail in ice core records (~mm scale or equivalent to subannual resolution). While this has enabled the study of past climates across a vast range of timescales, the full set of processes driving the highest frequency variability in these water isotope records remains poorly understood. In the EastGRIP ice core, we observe a strong relationship between high-frequency water isotope anomalies (sharp transitions on the scale of cms) and variability in the visual stratigraphy of the ice. The water isotope timeseries reveals these anomalies that would otherwise be missed using traditional lower resolution discrete sampling methods (5-50 cm scale). A comparison with the dark-field imaging of stratigraphic layers (high-resolution line-scanning system; 50µm/pix) from the EGRIP ice core indicates a correlation between bubble-free ice layers and the sharp transitions observed in the isotope record. Prior to this comparison, such anomalies in high-resolution isotope records were often dismissed as analytical artifacts. The striking correspondence to the bubble-free ice layers, which is a parameter measured independently from the isotopes, suggests the isotope variability is real. We are investigating a range of depositional and post-depositional processes that may may be able to explain the origin of this variability and its relationship to the physical properties of the ice. This study has implications for frequency analysis of the isotope data, and the related analysis of isotope diffusion and its effects on the recorded climate signal. Understanding these anomalies opens new doors to the interpretation of climate signals in ice cores.

Ice core water isotope records from Greenland and Antarctica are a valuable proxy for paleoclimate reconstruction, yet the processes influencing the climate signal stored in the isotopic composition of the snow are being challenged and revisited. Apart from precipitation input, post-depositional processes such as wind-driven redistribution and vapor–snow exchange processes at and below the surface are hypothesized to contribute to the isotope climate signal subsequently stored in the ice. Recent field studies have shown that surface snow isotopes vary between precipitation events and co-vary with vapor isotopes, which demonstrates that vapor–snow exchange is an important driving mechanism. Here we investigate how vapor–snow exchange processes influence the isotopic composition of the snowpack. Controlled laboratory experiments under forced sublimation show an increase in snow isotopic composition of up to 8 ‰ δ18O in the uppermost layer due to sublimation, with an attenuated signal down to 3 cm snow depth over the course of 4–6 d. This enrichment is accompanied by a decrease in the second-order parameter d-excess, indicating kinetic fractionation processes. Our observations confirm that sublimation alone can lead to a strong enrichment of stable water isotopes in surface snow and subsequent enrichment in the layers below. To compare laboratory experiments with realistic polar conditions, we completed four 2–3 d field experiments at the East Greenland Ice Core Project site (northeast Greenland) in summer 2019. High-resolution temporal sampling of both natural and isolated snow was conducted under clear-sky conditions and demonstrated that the snow isotopic composition changes on hourly timescales. A change of snow isotope content associated with sublimation is currently not implemented in isotope-enabled climate models and is not taken into account when interpreting ice core isotopic records. However, our results demonstrate that post-depositional processes such as sublimation contribute to the climate signal recorded in the water isotopes in surface snow, in both laboratory and field settings. This suggests that the ice core water isotope signal may effectively integrate across multiple parameters, and the ice core climate record should be interpreted as such, particularly in regions of low accumulation.

We report high resolution measurements of the stable isotope ratios of ancient ice (δ18O, δD) from the North Greenland Eemian deep ice core (NEEM, 77.45° N, 51.06° E). The record covers the period 8–130 ky b2k (y before 2000) with a temporal resolution of ≈0.5 and 7 y at the top and the bottom of the core respectively and contains important climate events such as the 8.2 ky event, the last glacial termination and a series of glacial stadials and interstadials. At its bottom part the record contains ice from the Eemian interglacial. Isotope ratios are calibrated on the SMOW/SLAP scale and reported on the GICC05 (Greenland Ice Core Chronology 2005) and AICC2012 (Antarctic Ice Core Chronology 2012) time scales interpolated accordingly. We also provide estimates for measurement precision and accuracy for both δ18O and δD., Measurement(s) isotope analysis • water ice core Technology Type(s) cavity ring-down spectroscopy Factor Type(s) δ18O • δD Sample Characteristic - Location Greenland Ice Sheet Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.14216441

An ice core drilled on the Renland Ice Cap in east-central Greenland contains a continuous climate record dating through the last glacial period. The Renland record is valuable because the coastal environment is more likely to reflect regional sea surface conditions, compared to inland Greenland ice cores that capture synoptic variability. Here we present the δ18O water isotope record for the Holocene, in which decadal-scale climate information is retained for the last 8 ka, and the annual water isotope signal is preserved throughout the last 2.6 ka. To investigate regional climate information preserved in the water isotope record, we apply spectral analysis techniques to a 300-year moving window to determine the mean strength of varying frequency bands through time. The strength of interannual frequency bands decays rapidly, but we find that the mean 15–20 year δ18O variability exhibits a millennial-scale cycle in line with the well-known Bond Cycle. Comparison to other North Atlantic proxy records suggests that the 15–20 year variability may reflect fluctuating sea ice conditions throughout the Holocene, driven by changes in the strength of the Atlantic Meridional Overturning Circulation. Additional analysis of the seasonal signal over the last 2.6 ka reveals that the winter δ18O signal has experienced a decreasing trend, while the summer signal has predominantly remained stable. The winter trend likely corresponds to an increase in Arctic sea ice cover, driven by a decrease in total annual insolation. In the context of anthropogenic climate change, the winter trend may have important implications for feedback processes as sea ice retreats in the Arctic.

The presence of large Northern Hemisphere ice sheets and reduced greenhouse gas concentrations during the Last Glacial Maximum fundamentally altered global ocean-atmosphere climate dynamics. Model simulations and palaeoclimate records suggest that glacial boundary conditions affected the El Niño-Southern Oscillation, a dominant source of short-term global climate variability. Yet little is known about changes in short-term climate variability at mid- to high latitudes. Here we use a high-resolution water isotope record from West Antarctica to demonstrate that interannual to decadal climate variability at high southern latitudes was almost twice as large at the Last Glacial Maximum as during the ensuing Holocene epoch (the past 11,700 years). Climate model simulations indicate that this increased variability reflects an increase in the teleconnection strength between the tropical Pacific and West Antarctica, owing to a shift in the mean location of tropical convection. This shift, in turn, can be attributed to the influence of topography and albedo of the North American ice sheets on atmospheric circulation. As the planet deglaciated, the largest and most abrupt decline in teleconnection strength occurred between approximately 16,000 years and 15,000 years ago, followed by a slower decline into the early Holocene.

Water isotopes in ice cores are used as a climate proxy for local temperature and regional atmospheric circulation as well as evaporative conditions in moisture source regions. Traditional measurements of water isotopes have been achieved using magnetic sector isotope ratio mass spectrometry (IRMS). However, a number of recent studies have shown that laser absorption spectrometry (LAS) performs as well or better than IRMS. The new LAS technology has been combined with continuous-flow analysis (CFA) to improve data density and sample throughput in numerous prior ice coring projects. Here, we present a comparable semi-automated LAS-CFA system for measuring high-resolution water isotopes of ice cores. We outline new methods for partitioning both system precision and mixing length into liquid and vapor components – useful measures for defining and improving the overall performance of the system. Critically, these methods take into account the uncertainty of depth registration that is not present in IRMS nor fully accounted for in other CFA studies. These analyses are achieved using samples from a South Pole firn core, a Greenland ice core, and the West Antarctic Ice Sheet (WAIS) Divide ice core. The measurement system utilizes a 16-position carousel contained in a freezer to consecutively deliver ∼ 1 m × 1.3 cm2 ice sticks to a temperature-controlled melt head, where the ice is converted to a continuous liquid stream and eventually vaporized using a concentric nebulizer for isotopic analysis. An integrated delivery system for water isotope standards is used for calibration to the Vienna Standard Mean Ocean Water (VSMOW) scale, and depth registration is achieved using a precise overhead laser distance device with an uncertainty of ±0.2 mm. As an added check on the system, we perform inter-lab LAS comparisons using WAIS Divide ice samples, a corroboratory step not taken in prior CFA studies. The overall results are important for substantiating data obtained from LAS-CFA systems, including optimizing liquid and vapor mixing lengths, determining melt rates for ice cores with different accumulation and thinning histories, and removing system-wide mixing effects that are convolved with the natural diffusional signal that results primarily from water molecule diffusion in the firn column.

We use a high-resolution water isotope record from the West Antarctic Ice Sheet Divide ice core (WDC) to evaluate the effects of water isotope diffusion for the last 29 ka B.P. Using spectral analysis of the data, we determine diffusion lengths in depth and time domains. The diffusion length quantifies the mean cumulative diffusive displacement of water molecules relative to their original location at time of deposition. We simulate the observed signal with models and find that our understanding of processes and conditions in the ice sheet is incomplete. With the effects of ice-deformational thinning removed, portions of the Holocene record show total diffusion lengths smaller than predicted for a lower limit case of diffusion through a single ice crystal. Such reduced diffusion is probably due to structural features such as crusts and tortuous porosity that inhibit vapor transport in the firn. In the late glacial portion of the record, diffusion lengths double between ~19.5 and 17 ka B.P. Known dependencies of diffusion on climatic variables do not account for this enhancement in models, and we hypothesize that it could arise from thermal gradients in the firn column, impurity-driven enhancement of solid ice diffusion, or changes in firn grain properties that alter vapor access to open pores. Despite model uncertainties, the WDC diffusion length chronology will be an essential input to future studies of high-frequency variability in the water isotope climate record, as it allows for the effects of diffusion to be removed.

The South Pole Ice Core (SPICEcore) was drilled in 2014–2016 to provide a detailed multi-proxy archive of paleoclimate conditions in East Antarctica during the Holocene and late Pleistocene. Interpretation of these records requires an accurate depth-age relationship. Here, we present the SP19 timescale for the age of the ice of SPICEcore. SP19 is synchronized to the WD2014 chronology from the West Antarctic Ice Sheet Divide (WAIS Divide) ice core using stratigraphic matching of 251 volcanic events. These events indicate an age of 54 302 ± 519 years BP (before the year 1950) at the bottom of SPICEcore. Annual layers identified in sodium and magnesium ions to 11 341 BP were used to interpolate between stratigraphic volcanic tie points, yielding an annually-resolved chronology through the Holocene. Estimated timescale uncertainty during the Holocene is less than 18 years relative to WD2014, with the exception of the interval between 1800 to 3100 BP when uncertainty estimates reach ± 25 years due to widely spaced volcanic tie points. Prior to the Holocene, uncertainties remain within 124 years relative to WD2014. Results show an average Holocene accumulation rate of 7.4 cm/yr (water equivalent). The time variability of accumulation rate is consistent with expectations for steady-state ice flow through the modern spatial pattern of accumulation rate. Time variations in nitrate concentration, nitrate seasonal amplitude, and δ15N of N2 in turn are as expected for the accumulation-rate variations. The highly variable yet well-constrained Holocene accumulation history at the site can help improve scientific understanding of deposition-sensitive climate proxies such as δ15N of N2 and photolyzed chemical compounds.

Several recent studies from both Greenland and Antarctica have reported significant changes in the water isotopic composition of near‐surface snow between precipitation events. These changes have been linked to isotopic exchange with atmospheric water vapor and sublimation‐induced fractionation, but the processes are poorly constrained by observations. Understanding and quantifying these processes are crucial to both the interpretation of ice core climate proxies and the formulation of isotope‐enabled general circulation models. Here, we present continuous measurements of the water isotopic composition in surface snow and atmospheric vapor together with near‐surface atmospheric turbulence and snow‐air latent and sensible heat fluxes, obtained at the East Greenland Ice‐Core Project drilling site in summer 2016. For two 4‐day‐long time periods, significant diurnal variations in atmospheric water isotopologues are observed. A model is developed to explore the impact of this variability on the surface snow isotopic composition. Our model suggests that the snow isotopic composition in the upper subcentimeter of the snow exhibits a diurnal variation with amplitudes in δ18O and δD of ~2.5‰ and ~13‰, respectively. As comparison, such changes correspond to 10–20% of the magnitude of seasonal changes in interior Greenland snow pack isotopes and of the change across a glacial‐interglacial transition. Importantly, our observation and model results suggest, that sublimation‐induced fractionation needs to be included in simulations of exchanges between the vapor and the snow surface on diurnal timescales during summer cloud‐free conditions in northeast Greenland., Key Points Data show evidence of isotopic fractionation during vapor exchange between the snow surface and atmosphereModel simulations suggest diurnal variation in the snow surface isotopic compositionWe propose an enhanced effective isotopic exchange between the surface of the snow grain and its interior

During the last glacial period, the North Atlantic region experienced a series of Dansgaard–Oeschger cycles in which climate abruptly alternated between warm and cold periods. Corresponding variations in Antarctic surface temperature were out of phase with their Northern Hemisphere counterparts. The temperature relationship between the hemispheres is commonly attributed to an interhemispheric redistribution of heat by the ocean overturning circulation. Changes in ocean heat transport should be accompanied by changes in atmospheric circulation to satisfy global energy budget constraints. Although changes in tropical atmospheric circulation linked to abrupt events in the Northern Hemisphere are well documented, evidence for predicted changes in the Southern Hemisphere’s atmospheric circulation during Dansgaard–Oeschger cycles is lacking. Here we use a high-resolution deuterium-excess record from West Antarctica to show that the latitude of the mean moisture source for Antarctic precipitation changed in phase with abrupt shifts in Northern Hemisphere climate, and significantly before Antarctic temperature change. This provides direct evidence that Southern Hemisphere mid-latitude storm tracks shifted within decades of abrupt changes in the North Atlantic, in parallel with meridional migrations of the intertropical convergence zone. We conclude that both oceanic and atmospheric processes, operating on different timescales, link the hemispheres during abrupt climate change. Abrupt glacial climate changes were slowly communicated between hemispheres by oceanic heat transport. Ice core data point to more rapid atmospheric teleconnections linking the North Atlantic, tropics, and southern storm track.

Permutation entropy techniques can be useful in identifying anomalies in paleoclimate data records, including noise, outliers, and post-processing issues. We demonstrate this using weighted and unweighted permutation entropy of water-isotope records in a deep polar ice core. In one region of these isotope records, our previous calculations revealed an abrupt change in the complexity of the traces: specifically, in the amount of new information that appeared at every time step. We conjectured that this effect was due to noise introduced by an older laboratory instrument. In this paper, we validate that conjecture by re-analyzing a section of the ice core using a more-advanced version of the laboratory instrument. The anomalous noise levels are absent from the permutation entropy traces of the new data. In other sections of the core, we show that permutation entropy techniques can be used to identify anomalies in the raw data that are not associated with climatic or glaciological processes, but rather effects occurring during field work, laboratory analysis, or data post-processing. These examples make it clear that permutation entropy is a useful forensic tool for identifying sections of data that require targeted re-analysis---and can even be useful in guiding that analysis., 15 pages, 7 figures

We asked young scientists: Are our schools and universities adequately prepared to educate young people for future challenges? What is the most pressing issue in your field, and what one improvement could your country make to its current education system to prepare students to face it? The responses expressed concerns about the current state of education in countries around the world. Many students lack access to the information they need, and those with access are often constrained by curriculum that emphasizes rote learning and isolated subjects. Our respondents suggested a variety of improvements to prepare the next generation for success.

Ice cores at Siple Dome, West Antarctic receive the majority of their precipitation from Pacific Ocean moisture sources. Pacific climate patterns, particularly in response to the El Niño-Southern Oscillation, affect local temperature, atmospheric circulation, snow accumulation, and water isotope signals at Siple Dome. We examined borehole temperatures, accumulation, and water isotopes from a number of shallow ice cores recovered from a 60 km north–south transect of the Dome. The data (with coverage from 1920–1995) reveal a microclimate heavily influenced by ENSO and the location of the Amundsen Sea Low Pressure Area. The Dome Summit and Pacific Flank respond to La Niña conditions by warming, increased isotope ratios, higher deuterium excess, and increased snowfall. The Inland Flank responds to El Niño conditions and cold interior air masses by cooling, decreased isotope ratios, lower deuterium excess, and decreased snowfall. ENSO-type spectral signatures (2–7 yr) are present in all water isotope records, but are not similar in their power structures. A longer 300 yr wavelet analysis record from the Dome Summit shows a late 19th-century climate shift similar to that seen in South Pacific coral isotope records. Our analyses suggest that while an ENSO signal is evident at Siple Dome, the microclimate effect makes climate reconstruction problematic, a conclusion which should be considered at other West Antarctic coastal dome locations.

Paleoclimate records are rich sources of information about the past history of the Earth system. Information theory provides a new means for studying these records. We demonstrate that weighted permutation entropy of water-isotope data from the West Antarctica Ice Sheet (WAIS) Divide ice core reveals meaningful climate signals in this record. We find that this measure correlates with accumulation (meters of ice equivalent per year) and may record the influence of geothermal heating effects in the deepest parts of the core. Dansgaard-Oeschger and Antarctic Isotope Maxima events, however, do not appear to leave strong signatures in the information record, suggesting that these abrupt warming events may actually be predictable features of the climate's dynamics. While the potential power of information theory in paleoclimatology is significant, the associated methods require well-dated and high-resolution data. The WAIS Divide core is the first paleoclimate record that can support this kind of analysis. As more high-resolution records become available, information theory could become a powerful forensic tool in paleoclimate science.

Water isotopes in ice cores are used as a climate proxy for local temperature and regional atmospheric circulation as well as evaporative conditions in moisture source regions. Traditional measurements of water isotopes have been achieved using magnetic sector isotope ratio mass spectrometry (IRMS). However, a number of recent studies have shown that laser absorption spectrometers (LAS) perform as well or better than IRMS. The new LAS technology has been combined with continuous flow analysis (CFA) to improve data density and sample throughput in numerous prior ice coring projects. Here, we present a comparable semi-automated LAS-CFA system for measuring high-resolution water isotopes of ice cores. We outline new methods for partitioning both system uncertainty and system mixing length into liquid and vapor components – useful measures for defining and improving the overall performance of the system. Critically, our methods take into account the uncertainty of depth registration that is not present in IRMS nor fully accounted for in other CFA studies. We also explain a method for introducing consecutive sections of isotopically distinct ice at the melt head to define the system-wide mixing length. These analyses are achieved using samples from a South Pole firn core, a Greenland ice core, and the WAIS Divide ice core. The measurement system utilizes a 16-position carousel contained in a freezer to consecutively deliver ~ 1 m × 1.3 cm2 ice sticks to a temperature controlled melt head, where the ice is converted to a continuous liquid stream, and eventually vaporized using a concentric nebulizer for isotopic analysis. An integrated delivery system for water isotope standards is used for calibration to the VSMOW-SLAP scale and depth registration is achieved using a precise overhead laser distance device with an uncertainty of ±0.2 mm. As an added check on our system, we perform inter-lab LAS comparisons using WAIS Divide ice samples, a corroboratory step not taken in prior CFA studies. The overall results are important for substantiating data obtained from LAS-CFA systems, including optimizing liquid and vapor mixing lengths, determining melt rates for ice cores with different accumulation and thinning histories, and removing system-wide mixing effects that are convolved with the natural diffusional signal that results primarily from water molecule diffusion in the firn column.

Paleoclimate records are extremely rich sources of information about the past history of the Earth system. We take an information-theoretic approach to analyzing data from the WAIS Divide ice core, the longest continuous and highest-resolution water isotope record yet recovered from Antarctica. We use weighted permutation entropy to calculate the Shannon entropy rate from these isotope measurements, which are proxies for a number of different climate variables, including the temperature at the time of deposition of the corresponding layer of the core. We find that the rate of information production in these measurements reveals issues with analysis instruments, even when those issues leave no visible traces in the raw data. These entropy calculations also allow us to identify a number of intervals in the data that may be of direct relevance to paleoclimate interpretation, and to form new conjectures about what is happening in those intervals—including periods of abrupt climate change.

The last glacial period exhibited abrupt Dansgaard-Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard-Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard-Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard-Oeschger dynamics.